The steel technology is rapidly changing with the introduction of bigger capacity Basic Oxygen Furnace, stringent conditions in ladle metallurgy with higher operating temperature. This needs superior quality basic refractories for high service performance. Magnesite and dolomite either tar or pitch bonded with graphite/carbon addition are traditionally used in these furnaces for achieving high slag corrosion resistance and superior refractory properties. Lime/calcia inspite of being strong contender of magnesite and dolomite is not used as a refractory material for its high tendency towards atmospheric hydration. When the refractoriness and thermodynamic stability in the presence of carbon is considered, lime would be even better refractory material than dolomite or magnesite in the steel making conditions. Non metallic inclusions are formed when an element dissolved in liquid steel reacts with refractory constituents like SiO.sub.2 /Cr.sub.2 O.sub.3 etc. of lining. The other advantage of lime refractories is its resistance to these dissolved elements, and thus formation of non metallic inclusions is reduced and this ultimately helps to improve the cleanliness of steel. The advantages of lime is utilized in some other countries by comixing it with dolomite/magnesite in brick making or cosintering at an early stage. Pure lime refractory is not used due to its perishing tendency. Still the lime will be a potential refractory if highly dense lime is produced, with an excellent hydration resistance and stability.
The basic raw material for production of sintered lime is natural limestone. High purity limestone is easily available in India and other parts of the world. Since the refractoriness of lime is very high, sintering of lime requires a high temperature which is not usually available in the industries. Therefore, the achievement of good quality sintered lime from purer natural lime stone lump needs a temperature above 2000.degree. C. The low flux lime stone (&lt;2% impurity) generates favourable liquid phase above this temperature. Whereas, high flux limestone (&gt;3% impurity) can be densified at a relatively lower temperature, but the product does not find use owing to its lower densification and low performance.
Reference may be made to the work of L. L. Wong and R. C. Bradt (1. Am. Ceram. Soc. Bull., 69 (7)1183-89 (1990) wherein it has been shown that impure limestone resulted poor densification at high firing temperature, 1600.degree. C.
L. Xintian et. al (2. Brit. Ceram. Trans. 93 (4)150-153 (1994) tried to develop CaO sinter with improved hydration resistance by the addition of A1.sub.2 0.sub.3. However, hydration resistance properties were inferior to the present work which is being reported herein.
Vezikova et. al (3. Refractories 33 (1-2)85-89 (1992) reported the development of lime sinters with the addition of TiO.sub.2. The material was fired at 1750.degree. C. which is much higher than the current work which is being sought to be protected.
Addink et. al (4. U.S. Pat. No. 4,795,725 (1989) developed a CaO refractory composition based on lime. To avoid hydration of lime Addink et. al used a thermosetting novolak type phenol resin in the batch so that it coates the CaO grains and prevent its hydration. The present work does not use any resin to protect the CaO grains.
Cassens (5. U.S. Pat. No. 4,463,100(1984)) developed a refractory material based on CaO by converting CaO into a different compound 2 CaO. SiO.sub.2.
Neville et. al (6. U.S. Pat. No. 4,843,044 (1989)) developed a lime based refractory material which contained lime sinter along with monosodium phosphate and alkali metal polyphosphate. No such binder has been used in the work reported by us.
For the improvement in the densification and hydration resistance, it is necessary to introduce mineralizer or additive in a reactive lime. This will help sintering at relatively lower temperature which improve the quality of the product to a great extent.